The invention relates to a method in connection with parallel-connected inverter modules, disclosed in the preamble of claim 1, and to an arrangement in connection with parallel-connected inverter modules, disclosed in the preamble of claim 5.
Inverters are used in frequency converters to generate a desired voltage or current to a load to be fed. A frequency converter typically receives its supply voltage from an AC network. This voltage is rectified in a rectifier unit and further converted to alternating voltage in the inverter to control the load in a desired manner. A conventional device fed from a frequency converter is a motor, which may be controlled in a reliable manner with the frequency converter using various control and adjustment principles.
As the power demand of motors increases, it is not necessarily reasonable to increase the size and capacity of individual frequency converters correspondingly. It is therefore known to connect frequency converter inverter modules in parallel to obtain the required output power.
Parallel-connection of inverter modules is typically achieved by connecting the same output phase of each parallel-connected module to a common phase supply. In other words, the output phases of each inverter module are connected to the corresponding output phases of other inverter modules. The switching instructions of the frequency converter are generated in a unit common to all the inverter modules, which unit then copies the instructions and sends them to each parallel-connected inverter module. Depending on the power needed, the number of parallel-connected modules may be anything from two upwards.
Inverter modules to be connected in parallel are typically fairly large and therefore placed in separate cabins. These cabins are cooled by blowing air into them to ensure proper operation and performance of the inverters. Today the temperatures of inverter output phases are measured for example directly from a power component of the output phase, or from the immediate vicinity thereof. The measured temperature is compared with a fixed temperature limit and thus, if the temperature rises above the limit, an overload possibly caused to the component is detected.
Prior art component temperature control thus relies on a single temperature measurement, but with this measurement only an overload of the component in question can be concluded with certainty. Other elements having an essential impact on the temperature of the switching component are properly functioning cooling and control of the switch component. Comparing the measured temperature with the fixed temperature limit does not, however, produce clear indication or diagnostic data of such disturbances that may occur.